The present invention relates to a scroll compressor incorporated in a reversible refrigerating cycle for an air conditioner, and more specifically, to a technique of switching between an internal high-pressure operation and an internal low-pressure operation and optimizing, during either operation, a thrust force that presses an orbiting scroll against a fixed scroll.
Scroll compressors comprise a refrigerant compressing section composed of a fixed scroll having a spiral wrap on a base plate and an orbiting scroll driven by an electric motor, the scrolls being engaged with each other. A low-pressure refrigerant that has completed its work during a refrigerating cycle is sucked from an outer peripheral side of the refrigerant compressing section, subsequently compressed as it approaches the center of the spiral, and then discharged as a high-pressure refrigerant from a discharge port formed in the center of the spiral.
During this refrigerant compressing operation, pressure from the interior of the refrigerant compressing section is always exerted on the orbiting scroll in a direction in which the pressure leaves the fixed scroll. Thus, such back pressure as resists the above pressure must be exerted on the orbiting scroll to prevent it from floating. A conventional example will be described with reference to FIG. 12.
In a basic configuration, the scroll compressor 1 comprises a cylindrical hermetic shell 2, and its interior is partitioned into a refrigerant discharge chamber 2a and a driving chamber 2b by means of a frame plate 3. The refrigerant discharge chamber 2a has a refrigerant compressing section 4 housed therein and composed of a fixed scroll 4a and an orbiting scroll 4b that are engaged with each other.
Although not shown, the driving chamber 2b has an electric motor housed therein and contains a predetermined amount of lubricant. A drive shaft 5 of the electric motor is extended through the frame plate 3 to the refrigerant compressing section 4 in such a manner that a crank shaft 5a at its tip is connected to the orbiting scroll 4b. The drive shaft 5 has a oil filler hole 5b drilled therein, and a rear surface of the orbiting scroll 4b and the driving chamber 2b are in communication with each other via the oil filler hole 5b. 
The scroll compressor 1 as a conventional example is of an internal high-pressure type; the refrigerant discharge chamber 2a and the driving chamber 2b are in communication with each other via a communication hole 6 that penetrates the fixed scroll 4a and the frame plate 3.
A low-pressure refrigerant that has completed its work during a refrigerating cycle (not shown) is sucked from an outer peripheral side of the refrigerant compressing section 4 through a sucking pipe 7a and then discharged as a high-pressure refrigerant from a discharge port 4c located in a central portion of the refrigerant compressing section 4. The refrigerant is guided from the refrigerant discharge chamber 2a through a discharge pipe 7b to a four-way switching valve (not shown), with part of the refrigerant flowing through the communication hole 6 into the driving chamber 2b. 
Thus, the pressure in the driving chamber 2b increases and the pressure on the rear surface of the orbiting scroll 4b also increases due to a flow via the oil filler hole 5b, but the internal pressure from the interior of the refrigerant compressing section 4 exerted on the orbiting scroll 4b is not uniform. That is, the apparatus exhibits a pressure gradient such that the pressure is lower on the outer peripheral side of the spiral (low-pressure refrigerant sucking side) and increases toward the center of the spiral.
To exert back pressure corresponding to this pressure gradient on the orbiting scroll 4b, in this conventional example, the rear surface side of the orbiting scroll 4b is separated into a main back pressure chamber 8a on the central portion side and a secondary back pressure chamber 8b on the peripheral portion side by means of a sealing 8 so that the high pressure in the driving chamber 2b is exerted in the main back pressure chamber 8a, while intermediate pressure, which is lower than the above high pressure, is exerted in the secondary back pressure chamber 8b via a throttle valve 9a. 
A check valve 9b is provided between the secondary back pressure chamber 8b and the outer peripheral side of the spiral of the refrigerant compressing section 4 so as to allow an excess of pressure to escape toward the refrigerant compressing section 4 if the intermediate pressure in the secondary back pressure chamber 8b reaches a predetermined value.
In addition to the internal high-pressure type described in the conventional example, the scroll compressor includes an internal low-pressure type that sucks the low-pressure refrigerant having completed its work during the refrigerating cycle, into the driving chamber 2b, from which the refrigerant is guided to the refrigerant compressing section 4.
Both in the internal high- and low-pressure types, the driving chamber (electric-motor chamber) is used as a circulating path for the refrigerant in order to prevent the electric motor from being overheated. Both of these types have the following advantages and disadvantages:
The internal high-pressure type is unlikely to have its performance significantly degraded by an overheated sucked gas, while the internal low-pressure type can be started up fast because an discharged gas is not cooled in the driving chamber during a heating operation.
In the internal low-pressure type, however, the lubricant in the compressor may be discharged to a heat exchanging circuit before being separated from a refrigerant gas, thus degrading the heat exchanging capability. Further, a sliding portion of the scroll may be seized due to an insufficient amount of lubricant in the compressor. In the internal low-pressure type, the sucked refrigerant gas is passed through the electric-motor chamber and overheated therein, so that its density is likely to decrease to degrade the performance.
The applicant has proposed a scroll compressor (for example, Japanese Patent Application published under Publication No. 2000-88386) that includes the advantages of both the internal high- and low-pressure types and that can switch between two operation modes in such a manner that the compressor operates as the internal high-pressure type during a cooling operation and as the internal low-pressure type during a heating operation.
In this scroll compressor, which can switch between the internal high- and low-pressure types, the pressure in the driving chamber varies depending on the operation mode, so that the above conventional method cannot exert a proper back pressure on the orbiting scroll. That is, during the internal low-pressure operation, the pressure in the driving chamber and thus the back pressure exerted on the orbiting scroll are low, possibly causing the orbiting scroll to be separated from the fixed scroll.
The present invention is adapted to solve the above problems, and its object is to provide a scroll compressor that allows a proper back pressure to be exerted on an orbiting scroll both during an internal high-pressure operation and during an internal low-pressure operation.
To attain this object, the present invention provides a scroll compressor comprising a hermetic shell having an interior partitioned into a refrigerant discharge chamber and a driving chamber by means of a frame plate, the refrigerant discharge chamber having a refrigerant compressing section housed therein and composed of a combination of a fixed scroll and an orbiting scroll, the driving chamber provided with an electric motor for driving the orbiting scroll, the scroll compressor being capable of switching between an internal high-pressure operation mode in which a high-pressure refrigerant generated in the refrigerant compressing section is transferred from the refrigerant discharge chamber through the driving chamber to a predetermined refrigerant circuit and an internal low-pressure operation mode in which the high-pressure refrigerant generated in the refrigerant compressing section is transferred from the refrigerant discharge chamber to the refrigerant circuit and in which a low-pressure refrigerant having completed its work is sucked into the refrigerant compressing section through the driving chamber, the scroll compressor including a first back pressure chamber arranged between a rear surface of a base plate of the orbiting scroll and the frame plate and which is in communication with the driving chamber to provide pressure from the driving chamber to the base plate of the orbiting scroll as back pressure, the scroll compressor being characterized by comprising a second back pressure chamber formed independently of the first back pressure chamber and back pressure control means for varying pressure in the second pressure chamber depending on the operation mode.
In the present invention, the back pressure control means provides such control as to set low pressure in the second back pressure chamber during the internal high-pressure operation mode, while setting high pressure during the internal low-pressure operation mode.
If the refrigerant circuit comprises a reversible refrigerating cycle including a four-way switching valve, an outdoor-side heat exchanger, an expansion valve, and an indoor-side heat exchanger, ad if, during the internal high-pressure operation mode, a refrigerant flows through the refrigerant discharge chamberxe2x86x92the four-way switching valve xe2x86x92the driving chamberxe2x86x92the outdoor-side heat exchangerxe2x86x92the expansion valvexe2x86x92the indoor-side heat exchangerxe2x86x92the four-way switching valvexe2x86x92the refrigerant compressing section, and during the internal low-pressure operation mode, it flows through the refrigerant discharge chamberxe2x86x92the four-way switching valvexe2x86x92the indoor-side heat exchanger xe2x86x92the expansion valvexe2x86x92the outdoor-side heat exchangerxe2x86x92the driving chamberxe2x86x92the four-way switching valvexe2x86x92the refrigerant compressing section, then the second back pressure chamber is connected to a pipe line between the four-way switching valve and the indoor-side heat exchanger by the back pressure control means.
Further, the back pressure control means may comprise a pressure responding valve that allows the second back pressure chamber to communicate with the refrigerant discharge chamber or a suction side of the refrigerant compressing section in response to the pressure in the driving chamber.
According to a preferred embodiment of the present invention, the pressure responding valve includes a valve chest drilled in the frame plate so that one end thereof is in communication with an interior of the driving chamber, while the other end thereof is in communication with the second back pressure chamber, and a slide valve arranged in the valve chest and moving in response to the pressure in the driving chamber. The valve chest has a first port in communication with the refrigerant discharge chamber and a second port in communication with the suction side of the refrigerant compressing section, the first and second ports being formed at different locations in an axial direction, and the slide valve has a communication hole that allows one of the above ports to communicate with the second back pressure chamber.
In this case, in order to stabilize the operation of the slide valve, a spring is preferably provided in the valve chest to urge the slide valve to the first port while the operation of the compressor is stopped, and the slide valve preferably comprises a valve disc comprising two portions of different diameters including a smaller diameter portion arranged at an end thereof closer to the second back pressure chamber so that the valve can be moved against an urging force of the spring on the basis of a difference in acting pressure between the two portions of the different diameters.
During a defrosting operation when the difference between the discharge pressure and suction pressure of the compressor decreases, the spring is preferably held on the first port by the spring.
The present invention is further characterized in that in order to form the second back pressure chamber, the frame plate has a first inner peripheral surface formed thereon and located closer to the refrigerant compressing section and a second inner peripheral surface formed thereon and located closer to the driving chamber and which has a smaller diameter than the first inner peripheral surface, in that a first thrust ring and a second thrust ring are provided between the frame plate and the refrigerant compressing section, the first thrust ring comprising a cylinder having one end surface in contact with the rear surface of the base plate of the orbiting scroll and an outer peripheral surface fitted on the first inner peripheral surface, the first thrust ring being movable in the axial direction, the second thrust ring comprising a cylinder having one end surface in contact with the rear surface of the base plate of the orbiting scroll and an outer peripheral surface fitted on the second inner peripheral surface, the second thrust ring being located inside the first thrust ring and being movable in the axial direction, and in that an interior of the second thrust ring forms the first back pressure chamber and a space surrounded by the first and second thrust rings forms the second back pressure chamber.
The second thrust ring maybe configured as one ring, but in order to enable the adjustment of an area in which back pressure acts, it preferably comprises two members including a base ring having one end surface in contact with the rear surface of the base plate of the orbiting scroll and having a reduced diameter portion at the other end, which has a reduced outer diameter, and a cylindrical sub-ring that is fitted on the reduced diameter portion of the base ring and that has an outer peripheral surface fitted on the second inner peripheral surface.
The fitting seal between each of the thrust rings and the frame plate can be managed based on the clearance there between, but an elastic seal ring with a U-shaped cross section or an O ring is preferably provided in a sliding surface of each of the thrust rings which comes into contact with the corresponding inner peripheral surface of the frame plate.
The present invention is further characterized in that in order to form the second back pressure chamber, the frame plate has a first inner peripheral surface formed thereon and located closer to the refrigerant compressing section and a second inner peripheral surface formed thereon and located closer to the driving chamber and which has a smaller diameter than the first inner peripheral surface, in that a thrust ring is provided between the frame plate and the refrigerant compressing section, the thrust ring having one end surface in contact with the rear surface of the base plate of the orbiting scroll and having a large-diameter seal portion fitted on the first inner peripheral portion and a small-diameter seal portion fitted on the second inner peripheral portion, and in that an interior of the thrust ring forms the first back pressure chamber and a space between those surfaces of the large- and small-diameter seal portions which are fitted on said frame plate forms the second back pressure chamber.
Also in this case, it is preferable that an elastic seal ring be provided in a fitting surface of each of the large- and small-diameter seal portions and that elastic means be provided between the thrust ring and the frame plate to urge the thrust ring to the rear surface of the base plate of the orbiting scroll. The elastic means is preferably a wave washer.
The present invention is further characterized in that in order to form the second back pressure chamber, the frame plate has a first inner peripheral surface formed thereon and located closer to the refrigerant compressing section and a second inner peripheral surface formed thereon and located closer to the driving chamber and which has a smaller diameter than the first inner peripheral surface, in that a thrust ring is provided between the frame plate and the refrigerant compressing section, the thrust ring having one end surface in contact with a rear surface of the base plate of the orbiting scroll and having a large-diameter seal portion fitted on the first inner peripheral portion and a small-diameter seal portion fitted on the second inner peripheral portion, in that an interior of the thrust ring forms the first back pressure chamber and the second back pressure chamber is formed between those surfaces of the large- and small-diameter seal portions which are fitted on the frame plate, and in that at least two rings including an inner ring and an outer ring are concentrically formed in the one end surface of the thrust ring which is in contact with the rear surface of the base plate of the orbiting scroll so that a space is formed between the inner ring and the outer ring, with the space and the second back pressure chamber in communication with each other via a communication hole. This configuration makes it possible to reduce the pressure on a sliding surface of the thrust ring relative to the orbiting scroll.
In this configuration, the inner ring preferably has an outer diameter smaller than that of the small-diameter seal portion. With this arrangement, even if a precession temporarily occurs in which the orbiting scroll is inclined from a horizontal surface relative to the fixed scroll, a pressing force sufficient to stably recover the original position can be obtained. For a similar reason, the area circumscribed by the inner diameter of the inner ring and the outer diameter of the outer ring is preferably smaller than the cross section of the second back pressure chamber.